1. Field of the Invention
This invention relates to ink jet recording and particularly to a magnetic ink jet printing apparatus and its method of operation.
2. Description of the Prior Art
In the well-known ink jet recorders, a jet recorder projects a continuous stream of ink drops of substantially uniform size and spacing along an initial trajectory toward a print medium. Dot matrix patterns such as alphanumeric characters are formed by selectively removing individual drops from the stream and controllably dispersing the remaining or print drops to be deposited on the print medium. The dispersion of the print drops to form the desired characters is performed concurrently with and in a direction orthogonal to the direction of relative motion of the jet recorder and the print medium.
The selective removal of drops from the stream involves applying a selection force of short duration to individual drops as they move toward the print medium. The selection force is generally orthogonal to the stream and to the direction of the dispersion of the print drops. Drops subjected to the selection force are deflected to follow a second trajectory that leads to a drop catcher which intercepts the ink drops in advance of the print medium.
One of the problems associated with drop selection is that the act of applying the selection force to the individual ink drop also acts to a somewhat lesser degree on adjacent drops. If the drops adjacent the selected drop are intended to be print drops, the selection force causes them to be likewise deflected from the initial trajectory since the generation of the selection force, particularly as it is practiced with field controllable ink drops such as ferrofluid ink drops, produces fringe effects. The fringe effect is particularly pronounced where the drops are relatively closely spaced for increased print rates and density of character impression. The undesirable aspect of uncontrolled fringing is that the print drops are deflected toward the trajectory of the unused drops thereby making removal of unused drops much more critical in the location of the drop catcher. Furthermore, where printing requires that two or more print drops in succession follow an unused and selected ink drop, the lead and trailing print drops are deflected by the fringe force whereas intermediate print drops are substantially unaffected. The net result is to have successive print drops following different trajectories. This results in printing of irregularly shaped characters on the print medium.
Various attempts have been made to eliminate or reduce the fringe effect. See for example, a publication by W. T. Pimbley entitled "Magnetic Transducer With Shunts For Magnetic Ink Jet Recorder," in the IBM Technical Disclosure Bulletin, on pages 3556-3557, Vol. 17, No. 12, May 1975, and a publication by D. C. Lo and J. W. Mitchell entitled "Modified Selector For Magnetic Ink Jet Printing," in the IBM Technical Disclosure Bulletin, on pages 3121-3122, Vol. 18, No. 9, February 1976. Such fringe compensators use fringe shields or fringe suppressors. While such devices can be effective in many applications, particularly in magnetic ink jet printers, it is not always possible or desirable to use additional magnetic structures. Such compensators increase the density of packaging as well as introduce problems of crosstalk.